At the same time as environmental regulations are mandating lower levels of sulfur and nitrogen in distillate fuels, refineries are being forced to process crude oils with larger amounts of these heteroatoms. In addition, residual S- and/or N-containing molecules can poison metal or acid sites on catalysts used downstream of a hydroprocessing process, such as in hydrocrackers. A need therefore exists to find catalysts which will do more efficient desulfurization and/or denitrogenation, particularly when existing hydroprocessing units are limited in their pressure capability.
Hydroprocessing catalysts usually comprise a sulfided Group 6 metal with one or more Group 8 to 10 metals as promoters on a refractory support, such as alumina. Bulk, unsupported catalysts are also known. Hydroprocessing catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally comprise molybdenum or tungsten sulfide promoted with a metal such as cobalt, nickel, iron, or a combination thereof. These sulfided catalysts generally have a layered or platelet morphology.
The ability to modify the nanostructural morphology of hydroprocessing catalysts appears to provide a possible way to control their activity and selectivity. Thus one of the important thrusts in hydroprocessing catalyst research appears to be the realization that a key synthesis tool for modifying nanostructure involves the incorporation of carbon into the sulfide structure. For example, U.S. Pat. No. 4,528,089 teaches that the use of carbon-containing catalyst precursors gives more active catalysts than catalysts prepared from sulfide precursors without organic groups. Use of organic impregnation aids in preparing oxide catalyst precursors has also been studied for some time (Kotter, M.; Riekeft, L.; Weyland, F.; Studies in Surface Science and Catalysis (1983), 16 (Prep. Catal. 3), 521-30, and U.S. Pat. No. 3,975,302).
In U.S. Pat. No. 7,591,942, it was demonstrated that sulfiding a bulk bimetallic Ni (or Co)/Mo (or W) phase containing a surfactant amine (located within the crystalline lattice of the oxide phase) with a backbone containing at least 10 carbon atoms gave a catalyst comprising stacked layers of MoS2 (or WS2) having a reduced number of stacks as compared to that obtained by sulfiding the carbon-free bulk oxide. A similar result was reported for bulk ternary Ni—Mo—W catalysts in U.S. Pat. No. 7,544,632. Lower number of stacks are important, since they may imply the presence of smaller crystals of Mo/W sulfides, which in turn can result in a larger surface area available for catalysis.
U.S. Published Patent Application No. 2007/0072765 discloses a method for preparing a catalyst composition, which method comprises: (a) impregnating an inorganic catalyst support with an aqueous solution containing (i) a salt of a Group VIII metal selected from Co and Ni, (ii) a salt of a Group VI metal selected from Mo and W, and (iii) an effective amount of an organic agent selected from amino alcohols and amino acids; (b) drying the impregnated catalyst support to remove substantially all water, thereby resulting in a metal-organic component on support catalyst precursor; (c) calcining the substantially dried catalyst precursor in the presence of an oxygen-containing atmosphere under conditions to oxidize at least 30%, but not all, of the organic agent and produce a partially oxidized catalyst precursor containing carbon; and (d) sulfiding the partially oxidized catalyst precursor in the presence of a sulfiding agent to produce a sulfided catalyst composition. Again the sulfide catalyst composition is found to have a lower number of stacks than equivalent compositions produced without organics present in the precursor.
Other potentially relevant publications can include, but are not limited to, U.S. Pat. Nos. 6,989,348 and 6,280,610, European Patent Nos. 0601722, 1041133, and 0181035, and International Publication Nos. WO 96/41848, WO 95/31280, WO 00/41810, and WO 00/41811.
Although, reducing number of stacks can be important in increasing catalyst surface area, it is not, in itself, sufficient to maximize catalyst activity, since it does not necessarily ensure that the promoter atoms (e.g., Co, Ni) are properly located on the sulfide stacks. According to the present invention, a new bulk mixed metal oxide catalyst precursor composition is provided which, when sulfided, not only reduces the number of stacks of the sulfided product but also enhances the efficiency of the promoter metal, thereby resulting in a catalyst of improved hydroprocessing activity.